The present invention relates to a Doppler type radio direction finder through which the direction of a remote source of RF emissions may be determined and, more specifically, to an improved aircraft borne direction finder or to a ground based Doppler direction finder of a stationary RF emitting source, irrespective of the orientation and position relative to the emitting RF source.
A Doppler direction finder is one which uses the relative motion between RF source and emitter to create a Doppler shift in received RF frequency to determine the direction of the RF source to the receiving antenna. The movement of the receiving antenna may be actual, for example, as by mechanical rotation of a boom or propeller that carries an antenna at the end of the respective boom or propeller, or if there is relative motion between the emitting RF source and the receiver, the antenna movement may be simulated by sequentially scanning or commutating an array of fixed antennas forming a closed loop. The motion of the receiving antenna normal to the approaching wavefronts of the emitting RF source impresses a shift in the apparent frequency of the incoming RF signal, the Doppler shift. When the antenna is approaching the RF source, the Doppler shift is positive, resulting in an increase in the frequency of the received signal, and while receding therefrom, the received signal frequency decreases to less than the actual frequency of the remote RF source.
Assuming constant angular velocity during the direction finding measurement, the magnitude of rotation of the receiving antenna relative to the phase of the resulting Doppler frequency shifts is a measure of the bearing or azimuth of the incoming RF. The maximum Doppler frequency shifts in each direction occurs when the antenna velocity toward or away from the signal source is a maximum; a positive maximum when moving directly toward the source of RF emissions, and a negative maximum when moving directly away. When the tangential antenna velocity is at right angles to the line connecting the RF source and the receiving antenna, the Doppler frequency shift will be zero. For example, if an antenna is rotated about a circular path in a horizontal plane at a constant speed, a maximum instantaneous frequency of the Doppler modulation occurs at the point at which the direction of propagation of the incoming RF is tangent to the circular path of the antenna, and the points of zero Doppler shift occur 90 degrees away from such maximum, where the direction of incident RF propagation is along a diameter of the foregoing circular path of rotation. The change in frequency is the product of maximum Doppler shift and a sinusoid due to the changing projection of the antenna velocity vector onto the line connecting the antenna and the RF emitting source.
Using those signals, prior Doppler direction finders display the azimuth of the moving RF source, as one example, by monitoring the phase of the Doppler shifted signal by synchronizing rotation of the deflection coil of a cathode ray tube with the rotation of the antenna. With the center of the cathode ray tube taken as the center of rotation and the vertical (or horizontal) calibrated to the zero azimuth position of the rotating antenna, the angle formed between a line drawn through the spot illuminated on phosphor of the cathode ray tube faceplate by the electron beam of the CRT, drawn to the center of the CRT and the vertical (or, alternately, horizontal) represents the azimuth of the source relative to the base angle of the receiving antenna.
It should be appreciated also that the maximum frequency deviation of the Doppler signal obtained depends upon the orientation of the emitting source that is being observed relative to the geometric plane of rotation of the antenna. If the RF is propagating in the same plane as the rotating antenna, the propagation vector of the emitted RF is entirely horizontal, and hence yields the maximum frequency deviation of Doppler signal at the Doppler system antenna. The foregoing occurs, as example, where the direction finder is installed in a helicopter flying at a certain height, and the antenna of the direction finder is located at an end of (and rotated in a circle by) the helicopter""s main rotor, defining the plane of rotation, and the source of RF emissions is an aircraft flying at that same height directly toward the helicopter. Alternatively if the aircraft is flying in parallel with the helicopter, a maximum frequency deviation will also be obtained, although the extent of that deviation will be less than in the former example.
If, however, the source aircraft is flying at a much greater height than the helicopter, the effective RF acted upon by the rotating antenna of the finder is only the horizontal component of the propagating RF, which has a smaller projection onto the RF propagation vector than the RF propagation vector. Hence, the Doppler shift derived is much less than before, although still sinusoidal with rotation and reaching smaller maxima of frequency shifts. And if the aircraft is flying directly overhead of the finder""s rotating antenna, the propagation vector is entirely vertical, zero degrees relative to the circular rotation path of the antenna, the horizontal component of that incoming RF is zero, and, hence, the maximum frequency deviation is zero, and no Doppler shift is observed. The foregoing variation in frequency deviation, thus, has been used to provide an indication of the elevation of the emitting source, simultaneously with an indication of the source azimuth.
Considering again the foregoing Cathode Ray Tube display, the greater the magnitude of the Doppler shifted signal, the greater the current produced in the rotating electromagnetic deflection coil of the CRT. That greater current produces a greater deflection of the electron beam, moving the illuminated spot a greater distance radially from the center. Thus the distance of the spot from the center in that direction finder system is a measure of the elevation of the RF emitting source, while the azimuthal position of the spot on the face of the CRT is a measure of the azimuth of that source. Such a direction finding system is described, as example, in U.S. Pat. No. 3,329,955 to Beukers et al, granted Jul. 4, 1967.
Another proposed Doppler direction finding system employs two antennas located at diametrically spaced positions about the axis of rotation, and combines the antenna outputs to achieve an enhanced signal. This is illustrated in U.S. Pat. No. 3,386,097 to Richter et al.
It is appreciated that an aircraft in which the foregoing Doppler radio direction finder may be installed, as example, does not always fly at a fixed horizontal attitude. Sometimes the aircraft may turn and bank, as example, and the plane of rotation of the rotating antenna, accordingly, will sometimes tilt from the horizontal (or vertical). Should the pilot of the observing aircraft make or be running a check for a source of RF emissions while the pilot""s craft is in a banked position, the emitting source may be undetectable if by chance, the emitting source is on another aircraft traveling vertically overhead of the plane of rotation of the Doppler antenna, as tilted, or if the relative horizontal component of emitted RF of that source is too small for the direction finder to meaningfully detect when the observing craft is banked.
None of the prior direction systems address the need to take a reading while the inspecting aircraft is banking. All such systems appear to rely upon the pilot of the craft maintaining the aircraft oriented with the circular path of the receiving antenna oriented in a plane parallel to the surface of the Earth. As becomes apparent, as an advantage, the present invention offers a solution to that problem.
The principal object of the present invention, therefore, is to enhance the effectiveness of Doppler direction finder systems.
And a further object is to minimize the effect of the relative orientation of the emitting source upon a Doppler direction finding system detection.
In accordance with the foregoing objects and advantages, the invention provides two antennas (or antenna pairs) that are rotated in two mutually orthogonal circular paths to intercept incoming RF from an emitting source, whose direction and/or elevation is to be determined. The antennas or antenna pairs are rotated about orthogonal axes at identical rotational velocities and in phase. The antennas thereby intercept propagating RF arriving from any direction.
Taking an extreme case as example, if the emitting source is directly overhead or below the center of the rotational plane of the one antenna (or antenna pair), as would result in no Doppler signal, the emitting source lies in the plane of rotation of the other antenna (or antenna pair), and would thereby result in maximum frequency of the Doppler signal. If the Doppler system is carried on an aircraft, as further example, and the aircraft is banking and, hence, might orient one antenna (or antenna pair) in a direction as results in minimal derived Doppler signal, the other antenna (or antenna pair) is simultaneously oriented to obtain maximal derived Doppler shift signal from the emission source. Alternatively, the two antennas (or antenna pairs) are used to respectively determine direction and elevation of the remote source of emissions.
In accordance with an additional feature of the invention, the received RF is processed digitally by electronic apparatus carried upon the propeller or other rotating arm that spins the antenna and the Doppler information is transmitted from that propeller as modulated RF to a receiver, demodulator and digital display unit located in a stationary position within the aircraft.
The foregoing and additional objects and advantages of the invention together with the structure characteristic thereof, which was only briefly summarized in the foregoing passages, becomes more apparent to those skilled in the art upon reading the detailed description of a preferred embodiment, which follows in this specification, taken together with the illustration thereof presented in the accompanying drawings.